Understanding spatiotemporal variation in net primary productivity (NPP) continues to be of fundamental importance to basic ecology and to applied conservation and management efforts for human well-being. Diversity is an important driver of NPP variation, but its effect is variable depending on ecosystem context and spatial scale as well as more closely linked to functional traits. Explicitly considering processes of trait-based community assembly and maintenance at relevant scales at which they occur can potentially resolve some of this variation. In my thesis, I address this gap by considering various processes that structure and maintain diversity in communities and analyze NPP as an outcome of these processes. I examine processes in high diversity tropical forests, relatively less explored in the context of biodiversity-ecosystem functioning. In Chapter 1, through a manipulated experiment, I show that diversity effect on biomass gain in seedling communities is modulated by light. This interactive effect holds across different functional groups that are known to respond uniquely to light. In Chapter 2, I expand the spatial scale of inquiry to community assembly processes that maintain diversity at regional spatial scales – dispersal and competition, akin to island biogeographic contexts. Through simulations, I show that dispersal and competition acting on correlated traits explain a range of variation in BEF curves observed in nature. Finally, in Chapter 3, expanding the focus of inquiry to include human aspects, I consider the impact of a biodiversity-driven human intervention, protected areas, in maintaining NPP. I show that at the landscape scale, mean annual NPP and temporal stability are both influenced by protection status, but the effectiveness of protection is contingent on environmental factors. Taken together, my thesis suggests that understanding the combined drivers of diversity and NPP can improve predictions for spatiotemporal variation of this ecosystem function. Further inquiry integrating diversity-gradients at multiple scales can improve process-based understanding of the effect of biodiversity on ecosystem functioning.

Thesis (M.Phil.) -- Griffith University, 2004. / Facsimile of the author's original dissertation. Pagination of document: x, 121 leaves. Includes bibliographical references. Also available online via the World Wide Web.

Tropical rainforests have an unparalleled capacity to sequester carbon, harbor biodiversity, and cycle water and nutrients due to their high rates of primary production. The large biomass stocks and rapid regeneration rates of these forests are often attributed to ample soil nitrogen and quick recovery of the nitrogen cycle in tropical soils following disturbance. Symbiotic nitrogen-fixing trees, which are relatively abundant at tropical latitudes, have the greatest capacity to provide tropical rainforests with new nitrogen, yet the ecological drivers of tropical symbiotic nitrogen fixers and their effects on the forests they inhabit are not well understood. This dissertation consists of four chapters that examine the patterns, environmental controls, and ecological consequences of symbiotic nitrogen-fixing trees in regenerating and intact rainforests in the Caribbean lowlands of Costa Rica. In chapter 1, I use field sampling in a chronosequence of rainforest plots to show that symbiotic nitrogen fixation declines through succession despite increases in the basal area of nitrogen-fixing trees. Chapters 2 and 3 describe results from a controlled shadehouse experiment assessing the effects of light, soil nitrogen, and plant competition on nitrogen fixation rates and the growth and biomass allocation of nitrogen fixers and non-fixers. In chapter 2, I demonstrate that light regulates nitrogen fixation more strongly than soil nitrogen availability. This is a departure from the historical focus on soil nitrogen as the primary regulator of nitrogen fixation and has the potential to resolve longstanding paradoxes of tropical nitrogen cycling. In chapter 3, I show that nitrogen fixation provides some resistance to competitive effects from neighboring plants in nitrogen-limited conditions, and that nitrogen fixers in these conditions downregulate their fixation rates in the presence of a competitor. This chapter also demonstrates that nitrogen fixation does not represent a significant structural cost to the plant, as reduced root biomass of nitrogen fixers more than compensates for allocation to nodule production. Finally, in Chapter 4, I demonstrate that nitrogen-fixing trees in our chronosequence plots do not promote forest growth, as expected given their capacity to fertilize their neighbors, but rather inhibit forest growth because they are strong competitors. These chapters describe several unexpected findings – i.e. that light primarily drives nitrogen fixation and that nitrogen fixers slow forest growth – which provide new and important insight into the role that nitrogen-fixing trees play in the growth of Costa Rican rainforests.

Typescript. Copies of two papers co-authored by the author, in back cover pocket. Bibliography: leaves 128-143.

Thesis (doctoral)--Swedish University of Agricultural Sciences, 2002. / Abstract inserted. Appendix reprints four papers and manuscripts co-authored with others. Includes bibliographical references. Also partially issued electronically via World Wide Web in PDF format; online version lacks appendix.

Thesis (Ph. D.)--University of Oregon, 2003. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 255-270). Also available for download via the World Wide Web; free to University of Oregon users.

Typescript / Copies of two papers co-authored by the author, in back cover pocket / Bibliography: leaves 128-143 / 143 leaves, [60] leaves : ill., maps ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Botany, 1987

Thesis (Ph. D.)--University of Western Sydney, Hawkesbury, 1996. / Includes bibliographical references (p. 511-557).

Thesis (M.S.)--University of Puerto Rico, Mayagüez Campus, 2004. / Tables. Printout. Abstract in English and Spanish. Includes bibliographical references (leaves 30-38).

Thesis (doctoral)--Universiteit van Amsterdam, 1999. / Includes bibliographical references (p. 138-152).